Conventionally, in an engine, there is a demand for reducing energy loss i.e. cooling loss accompanied by release of heat energy of combustion gas from a wall surface of a combustion chamber to the outside of the engine in order to enhance fuel economy.
For instance, Japanese Unexamined Patent Publication No. 2013-194712 discloses an engine, in which a combustion speed of a fuel-air mixture is increased by adding ozone to intake air introduced to a combustion chamber so as to terminate combustion before flame reaches a wall surface of the combustion chamber. With use of the engine, it is possible to suppress heat energy from being released to the outside of the engine via a wall surface of a combustion chamber due to contact of high temperature flame with the wall surface of the combustion chamber.
In the engine disclosed in Japanese Unexamined Patent Publication No. 2013-194712, it is necessary to install an ozone generation device within an intake pipe or the like in order to add ozone to intake air. This may complicate the structure of the engine and may be disadvantageous in terms of cost. In view of the above, there is a demand for reducing cooling loss with a simplified configuration. Further, also in the engine disclosed in Japanese Unexamined Patent Publication No. 2013-194712, in a case where intake air flows along a wall surface of a combustion chamber within the combustion chamber before start of combustion, a fuel-air mixture is generated in the vicinity of the wall surface of the combustion chamber, and consequently, it is not possible to sufficiently avoid contact of combustion gas with the wall surface of the combustion chamber.